Calibration of process control temperature transmitter

ABSTRACT

A transmitter in a process control loop measures temperature. A calibrator includes a known calibration element which is connected to the transmitter. Software in the transmitter compares a measured value of the calibration element with the actual value of the calibration element and responsively calibrates the transmitter. The calibrator includes a temperature calibration sensor for coupling to a terminal block of the transmitter. The temperature calibration sensor provides an actual temperature input to the transmitter. The transmitter measures actual temperature of the terminal block and compares actual temperature with a temperature measured by an internal terminal block temperature sensor, and responsively calibrates the internal temperature sensor.

This is a Divisional of application Ser. No. 08/313,452, filed Sep. 27,1994, now U.S. Pat. No. 5,669,713. Priority of the prior application isclaimed pursuant to 35 USC § 120.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

BACKGROUND OF THE INVENTION

The present invention relates to transmitters used in process controlloops. More specifically, the present invention relates to calibratingtemperature transmitters used in process control loops.

Process control transmitters are used to measure process parameters in aprocess control system. Microprocessor based transmitters include asensor or sensor input, an analog-to-digital converter for converting anoutput from the sensor into a digital format, a microprocessor forcompensating the digitized output, and an output circuit fortransmitting the compensated output. Typically, this transmission isover a process control loop, such as a 4-20 mA current loop. One exampleparameter is temperature, which is sensed by measuring the resistance ofan RTD (Resistive Temperature Device, also called a PRT, PlatinumResistance Thermometer) sensor, or the voltage output of a thermocouplesensor.

One technique for sensing the parameter is by comparison of theresistance of an RTD with an internal reference resistance level in thetransmitter which is calibrated when the transmitter is manufactured.For example, resistance of the RTD is measured by connecting it inseries with a known reference resistance (R_(REF)) and applying acurrent common to both resistances. The resistance of the sensor(R_(INPUT)) is expressed as follows: ##EQU1## where: R_(REFNOM) =thenominal resistance of the reference resistance in ohms;

R_(CAL1) =a calibration offset in ohms determined during manufacture.This value represents the difference between the calibrated referenceresistance (R_(REF)) and the nominal value of the reference resistance;

R_(CAL2) =a user calibration offset;

V_(RINPUT) =voltage drop across the input; and

V_(RREF) =voltage drop across R_(REF).

Equation 1 is used as part of an auto-zeroing and auto-spanning routinein that errors in measurement of V_(RINPUT) in the numerator tend to bethe same as errors in V_(RREF) in the denominator and therefore cancel.

The A/D converter digitizes the voltages of Equation 1. Themicroprocessor receives the digitized values and calculates andcompensates R_(INPUT) according to Equation 1 which is converted into acorresponding sensor temperature value with a look-up table or suitableequation by the microprocessor. The output circuit in the transmitterreceives the sensor temperature value and provides an output to the loopas a current level or as a digital value. Unfortunately, R_(REF)sometimes drifts from the calibrated value of R_(REFNOM) and R_(CAL1),leading to inaccuracies in measurement of temperature.

A typical prior art method of calibrating the R_(REF) is to connect anexternal predetermined resistance to the sensor input of the transmitterand have the transmitter enter a calibration mode. The transmittercompares the expected value of the predetermined resistance with themeasured value of the R_(INPUT) and uses the difference to calibrate itselectronics by adjusting R_(CAL2), the user trim. Typically, thepredetermined resistance is an active circuit or a resistance decadebox. Active circuitry which simulates resistance is usually inaccurateand typically incompatible with an intrinsically safe environment.Decade boxes are unrepeatable and inaccurate relative to precisiontransmitters and are unwieldy, requiring the transmitter to bedisconnected from the process loop in the field and brought into acalibration lab. They also suffer from drift in resistance value fromchanges in ambient temperature.

Thermocouples are also used by transmitters to measure temperature. U.S.Pat. No. 4,936,690, entitled "Thermocouple Transmitter with ColdJunction Compensation," and assigned to the same assignee as the presentapplication, describes such a measurement. A thermocouple junctiongenerates a voltage across the junction related to its temperature.However, a junction between dissimilar metals at the terminal block ofthe transmitter introduces another thermocouple voltage. To measure thethermocouple temperature, it is necessary to measure the temperature ofthis "cold junction" at the terminal block connection and compensate forthis voltage. Typically, in microprocessor based transmitters, atemperature sensor is thermally coupled to the terminal block andconnected to the microprocessor of the transmitter through the A/Dconverter. The microprocessor compensates the thermocouple voltage basedupon the cold junction temperature measured by the PRT. Errors inthermocouple temperature measurements arise if the transmitterinaccurately measures the temperature at the cold junction.

Prior art attempts at calibrating thermocouple multiple inputtransmitters focus on calibrating the transmitter with a known voltagesource. However, the source has typically been relatively unstable withrespect to a precision transmitter. For example, a precisionthermocouple transmitter has a typical accuracy of 0.04 percent of theactual value. Further, little attention has been given to calibratingthe cold junction temperature measurement.

The art lacks a self-contained calibration device for adequatelycalibrating temperature measurements. Further, the art lacks a devicethat is stable enough to be effectively traceable to a NIST (NationalInstitute of Standards Technology) traceable reference within theprecision of a high precision transmitter. As with RTD'S, voltagecalibrations are inaccurate and not stable enough to be useful as NISTtraceable devices (e.g., they are not stable enough to hold theircalibration for very long).

SUMMARY OF THE INVENTION

The present invention is a temperature transmitter which transmits ameasured temperature over a process control loop. To calibrate thetransmitter, a known NIST traceable resistance R_(NIST) is coupled tothe input of the transmitter, which the transmitter measures asR_(INPUT). The NIST calibrated value of R_(NIST) (R_(NISTCAL)) isprovided to the microprocessor of the transmitter which thenresponsively calculates a calibration value R_(CAL3) for use insubsequent measurement of R_(INPUT) and temperature. In a differentcalibration mode, the transmitter is coupled to a known NIST traceablevoltage reference V_(NIST) which the transmitter measures. The NISTcalibrated value of V_(NIST) (V_(NISTCAL)) is provided to thetransmitter input circuitry and the transmitter responsively calculatesa voltage calibration value V_(CAL3). In a final calibration mode, atemperature probe that may have NIST traceable calibration is coupled tothe terminal block of the transmitter, and the transmitter measures thecold junction temperature and then responsively calibrates an internalcold junction temperature sensor.

A calibrator adapted for coupling to the transmitter includes the NISTtraceable resistance R_(NIST), the NIST traceable voltage V_(NIST) andthe NIST traceable temperature probe (P_(NIST)). In one embodiment, thecalibrator communicates with the transmitter over the two-wire processcontrol loop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a temperature transmitter connected tomeasure temperature with an RTD sensor.

FIG. 1B is a block diagram, like FIG. 1A, of a temperature transmitterconnected to measure temperature with a thermocouple sensor.

FIG. 2 is a block diagram of one embodiment of a calibrator.

FIG. 3 is a block diagram of another embodiment of a calibrator.

FIG. 4 is a diagram which shows a calibrator coupled to a transmitter.

FIG. 5 is a flow chart of operation of a microprocessor in thetemperature transmitter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A is a block diagram of temperature transmitter 10 connected tomeasure temperature with an RTD sensor. Transmitter 10 couples toprocess control loop 12 which provides power to transmitter 10 and overwhich information is transmitted and received. Transmitter 10 includesterminal block 14 having terminals 1 through 4 for coupling to, forexample, an RTD temperature sensor 16 or a thermocouple temperaturesensor 18 (shown in FIG. 1B). FIG. 1A shows the electrical connectionsto RTD 16. Sensor 16 (and sensor 18) can be either internal or externalto transmitter 10. Transmitter 10 includes multiplexer 20 controlled bymicroprocessor 22 which is coupled to control loop 12 throughinput/output (I/O) circuitry 24. Multiplexer 20 multiplexes appropriatesets of analog signals, including signals from terminals 1 through 4, topositive and negative inputs of differential amplifier 26, whichconnects to high accuracy A/D converter 28. In one embodiment, converter28 has an accuracy of 17 bits and a conversion rate of 14samples/second. One example conversion technique is described in"Resolve 22 Bits Easily with Charge-Balance ADC's," by Thomas J. Mego,published Jun. 25, 1987, in Electronic Design Magazine, page 109. Memory30 stores instructions and information for microprocessor 22, whichoperates at a speed determined by clock 32. Multiplexer 20 selectivelyconnects its inputs to the positive or negative inputs of differentialamplifier 26. A resistance 38 couples to multiplexer 20 and is connectedin series with RTD 16.

In operation, transmitter 10 measures temperature of sensor 16 andtransmits a representation of temperature over control loop 12.Transmitter 10 employs the following equation to compute the temperatureof RTD 16: ##EQU2## where: R_(REFNOM) =the nominal resistance of thereference resistance in ohms, and/or stored in memory 30;

R_(CAL1) =a calibration offset to R_(REFNOM) in ohms determined duringmanufacture, and/or stored in memory 30. This value represents thedifference between the actual value of the reference resistance(R_(REF)) and the nominal value of the reference resistance atmanufacture;

R_(CAL2) =a user calibration offset to R_(REFNOM) stored in memory 30;

R_(CAL3) =a field calibrated offset to R_(REFNOM) in accordance with oneaspect of the invention, stored in memory 30;

V_(RINPUT) =voltage drop across the input; and

V_(RREF) =voltage drop across R_(REF).

Current source 50 provides current I_(S) through sensor 16 (viaterminals 1 and 4) and reference resistor 38. Microprocessor 22 measuresthe voltage drop (V_(RINPUT)) across RTD 16 between terminals 2 and 3,and the voltage drop (V_(RREF)) across resistor 38 with MUX 20.R_(REFNOM), R_(RCAL1), R_(RCAL2) and R_(RCAL3) are retrieved from memory30. R_(CAL3) is used to accurately calibrate the measurement to a NISTstandard as described below. In a four-wire resistance measurement suchas this, the voltage drop across the connections to terminals 2 and 3 islargely eliminated, since substantially all the current flows betweenterminals 1 and 4, and has little impact on the accuracy of themeasurement. R_(INPUT) is converted to temperature units with a look-uptable or suitable equation stored in memory 30. The improved accuracy ofthe invention is partially achieved by using additional calibration datastored in memory and additional computational power of themicroprocessor.

FIG. 1B shows transmitter 10 connected to measure temperature withthermocouple sensor 18 which creates a voltage V_(TCINPUT) acrossterminals 1 and 2. Multiplexer 20 couples inputs of differentialamplifier 26 to terminals 2 and 1. FIG. 1B shows a voltage reference(V_(TCREF)) 36 coupled to MUX 20 and current source 50. A voltagereference 36 includes resistors 40 and 42 connected as a voltage dividerand a Zener diode 44.

Transmitter 10 measures the temperature of thermocouple sensor 18 bydetermining the thermocouple voltage V_(TC) with the following equation:##EQU3## where: V_(TCINPUT) =the measured voltage across terminals 1 and2 of terminal block 14 sensed by amplifier 26;

V_(TCREF) =the measured voltage generated by voltage reference 36 assensed by amplifier 26;

V_(TCREFNOM) =a nominal value of voltage reference 36 stored in memory30;

V_(CAL1) =a factory calibration offset to V_(TCREFNOM) placed in memory30 during manufacture of transmitter 10;

V_(CAL2) =a user trim calibration offset to V_(TCREFNOM) stored inmemory 30; and

V_(CAL3) =a calibration offset to V_(TCREFNOM) stored in memory 30 forcalibrating transmitter 10 to a NIST traceable voltage reference.

However, the junction between two dissimilar metals formed at terminal 1introduces a cold junction error voltage proportional to the temperatureof the junction. The error voltage is sensed by measuring thetemperature of the junction at terminal 1 based upon the resistance of aPRT (Platinum Resistance Thermometer) sensor 34 and subsequently using astandard equation or look-up table to determine the cold junction errorvoltage. Resistance of sensor 34 is measured using Equation 2 byapplying a current I_(S) from with current source 50. The resistance ofPRT sensor 34 is calculated as:

    R.sub.CJCPRT =R.sub.PRTEQ.2 +R.sub.PRTCAL                  Equation 4

where:

R_(CJCPRT) =the calibrated resistance of PRT sensor 34;

R_(PRTEQ).2 =the resistance of PRT as calculated with Equation 2; and

R_(PRTCAL) =a calibration offset stored in memory 30.

Microprocessor 20 computes V_(TC) according to Equation 3 and R_(CJCPRT)according to Equation 4, then effectively subtracts R_(CJCPRT) fromV_(TC) using an appropriate look-up table or equation. Then theresulting compensated temperature of sensor 18 is coupled to loop 12 viaoutput circuitry 24.

FIG. 2 is a schematic diagram of a calibrator 60 in accordance with theinvention. Calibrator 60 includes a voltage reference (V_(NIST)) 62,resistor reference (R_(NIST)) 64, compound switch 66, connectors 68a,68b and 68c, and connector/probe 70 which includes a NIST traceable PRTsensor 72. R_(NIST) should be selected as a high accuracy resistancewhich is stable over time and temperature. Similarly, V_(NIST) should bean accurate voltage reference stable over time and temperature, such asa voltage drop across a temperature and time stable diode. Switch 66includes ganged switches 66a through 66d which are operated in unisonand selectively couple connectors 68a through 68c and connector/probe 70to reference resistor 64, voltage reference 62 and PRT sensor 72,respectively. Probe 70 is adapted for thermally coupling to terminalblock 14 of transmitter 10. In one embodiment, probe 70 screws intoterminal block 14.

In operation, V_(NIST) 62 and R_(NIST) 64 and probe 70 are calibrated toa NIST traceable reference using a standard laboratory NIST traceablevolt/ohm meter and a temperature standard calibrated 0° C. ice point.The precise calibrated values of reference voltage 62 (V_(NISTCAL)),reference resistance 64 (R_(NISTCAL)) and P_(NISTCAL) (NIST calibratedvalue of R_(PROBE) at 0° C.) are marked on labeling 74 on calibrator 60.Calibrator 60 is taken to the location of transmitter 10 and connectors70, 68c, 68b and 68a are connected to terminals 1, 2, 3 and 4,respectively. Switch 66 is operated to connect one end of resistorR_(NIST) 64 to terminals 1 and 2 and terminal block 14. In thisposition, R_(NIST) 64 is connected in a manner similar to RTD 16, shownin FIG. 1A. Transmitter 10 receives a command (such as a HART® command)over loop 12 which instructs transmitter 10 to enter a calibration modeand an operator provides transmitter 10 with the NIST calibrated value(R_(NISTCAL)) of R_(NIST) 64, as indicated on labeling 74. Thisinformation is provided to transmitter 10, for example, over two-wireloop 12 using a hand held communicator such as that described in U.S.Pat. No. 4,988,990, entitled "Dual Master Implied Token CommunicationSystem." Microprocessor 22 causes V_(RINPUT) and V_(REFNOM) to be sensedand digitized, and calculates a new value of R_(CAL3), R_(CAL3)(NEW), asfollows: ##EQU4## R_(CAL3)(NEW) is stored in memory 30 as R_(CAL3) forsubsequent measurements.

Next, the operator positions switch 66 to couple voltage referenceV_(NIST) 62 between terminals 1 and 2 of terminal block 14 asV_(TCINPUT). The NIST calibrated value (V_(NISTCAL)) of the V_(NIST) 62is provided to transmitter 10 over 12. A new value of V_(CAL3)(V_(CAL3)(NEW)) is determined: ##EQU5## V_(CAL3)(NEW) is stored inmemory 30 for subsequent use as V_(CAL3).

After calibrating resistance and voltage measurements, cold junctiontemperature sensor 34 is calibrated by placing switch 66 in position tocouple the other end of PRT sensor 72 to terminals 1 and 2, and theother end to terminals 3 and 4. Probe/connector 70 is thermally coupledto terminal 1 proximate sensor 34 such that the temperature of PRTsensor 72 is substantially the same as the temperature of PRT internalsensor 34. For example, probe 70 can screw into terminal 1. Transmitter10 receives the probe NIST value P_(NISTCAL) and initiates PRTcalibration by measuring resistance R_(PROBE) of PRT sensor 72 andresistance R_(CJCPRT) PRT sensor 34 and by using Equation 2. In oneembodiment, microprocessor 22 compares the difference in thesemeasurements which represents the amount sensor 34 has drifted from itspreviously calibrated value. This provides a new cold junctioncalibration value R_(PRTCAL)(NEW) as follows:

    R.sub.PRTCAL(NEW) =R.sub.PROBE -R.sub.CJCPRT +(P.sub.NOM -P.sub.NIST)Equation 7

where P_(NOM) is the nominal resistance of a standard PRT at 0° C.R_(PRTCAL)(NEW) is stored in memory 30 as R_(PRTCAL).

FIG. 3 is a block diagram of a calibrator 80 in accordance with anotherembodiment. Calibrator 80 includes reference resistance (R_(NIST)) 64,voltage reference (V_(NIST)) 62, NIST traceable probe P_(NIST) 70, andconnectors 68a, 68b, 68c and 70. Microprocessor 82 controls calibrator80. Microprocessor 82 displays information on display 84, storesinformation in memory 88, receives information from an operator throughkeypad 90, and communicates over two-wire loop 12 through input/outputcircuitry 92. Microprocessor 82 selectively couples R_(NIST) 64,V_(NIST) 62 and PRT sensor 72 (not shown in FIG. 3) to connectors 68athrough 68c and 70 using multiplexer 94 which operates similarly toswitch 66, shown in FIG. 2.

In operation, an operator measures references 62 and 64 and probe 70relative to a NIST traceable volt/ohm meter and temperature standard,and enters these values (R_(NISTCAL), V_(NISTCAL) and P_(NISTCAL)) intomemory 88 using microprocessor 82 and keypad 90. Calibrator 80 isconnected to transmitter 10, through connectors 68a through 68c and 70,and couples to loop 12 through input/output circuitry 92 so thatcalibrator 80 communicates directly with transmitter 10 pursuant tostandard communication techniques such as the HART® protocol. Anoperator instructs calibrator 80 to begin calibration by entering acommand on keypad 90. Microprocessor 82 controls multiplexer 94 toconnect voltage reference 64 to transmitter 10. Microprocessor 82instructs microprocessor 22 of transmitter 10 to enter a calibrationmode by sending an appropriate command through I/O circuitry 92 overtwo-wire loop 12. Microprocessor 82 sequentially connects resistance 64and voltage reference 62 to transmitter 10 through MUX 94 and instructsmicroprocessor 20 to calibrate resistance and voltage, as disclosedabove. Temperature sensor 34 in terminal block 14 is calibrated usingsensor 72 (not shown in FIG. 3) which is coupled to transmitter 10through multiplexer 94 and R_(PRTCAL)(NEW) is determined. In oneembodiment, transmitter 10 communicates the amount that its referencevalues have drifted for display in accordance with ISO 9000requirements. This can be displayed on a hand held communicator ordisplayed on calibrator 80.

FIG. 4 is a block diagram of a process control system 100 includingtransmitter 10, loop 12, and controller electronics 102 which aremodeled as resistance 104 and voltage source 106. FIG. 4 showscalibrator 80 coupled to loop 12 and to RTD sensor 16 through junctionbox 108 which receives a connector 110. Junction box 108 is analternative to individually screwing connectors 68a, 68b, 68C and 70into terminal block 14. Junction box 108 allows calibrator 90 todirectly couple to terminal block 14 through connector 110. Pluggingconnector 110 into junction box 108 actuates switches 112, therebyproviding electrical connections to terminals 1 through 4 of terminalblock 14 and electrically disconnecting sensor 16. To perform a coldjunction temperature sensor calibration, sensor 70 is attached toterminal 1 of terminal block 14. Junction box 108 provides a simple andconvenient technique of accessing terminals 1 through 4 of terminalblock 14. As resistance measurements are made using a four-wireconnection, resistance introduced through switches 112 does not reducethe calibration accuracy.

FIG. 5 is a flow chart 140 showing operation of microprocessor 22according to instructions in memory 30. Flow chart 140 normally operatesin loop 142 which includes measuring and transmitting temperature overtwo-wire loop 12 at block 144 and checking for a calibration initiationcommand received over loop 12 at block 146. Upon receipt of acalibration command, microprocessor 22 begins calibration at block 148.

Determination of R_(CAL3)(NEW) is initiated at block 148. Prior tosending a command to begin calibration, an operator has connected theNIST traceable resistance to terminal block 14. At block 148,microprocessor 22 measures and transmits R_(NIST) over process controlloop 12. The transmitted value of R_(NIST) can be displayed on a handheld communicator or on calibrator 80. At block 150, microprocessor 22obtains the value of R_(NISTCAL) over loop 12 from calibrator 80. Atblock 152, the values of R_(INPUT) and R_(REF) are measured and a newcalibration value R_(CAL3)(NEW) is calculated and stored at block 154,according to Equation 5. At block 156, R_(NIST) is again measured andits value is transmitted over loop 12 for receipt by a hand held unit orby calibrator 80. This value should be approximately the same as thevalue of R_(NISTCAL).

Next, an operator connects the NIST traceable voltage reference toterminal block 14 for calculation of V_(CAL3)(NEW). V_(NIST) is measuredand transmitted over two-wire loop at block 158. Microprocessor 22obtains V_(NISTCAL) from calibrator 80 over loop 12 and V_(TCINPUT) andV_(TCREF) at block 160. At block 162, a value of V_(CAL3)(NEW) iscalculated according to Equation 6 and stored in memory. V_(NIST) isagain measured and its value is transmitted over loop 12 at block 164.This value should be approximately the same as V_(NISTCAL). Thiscompletes calibration of voltage measurement, and probe 70 is connectedto terminal block 14 for calibration of PRT sensor 34. At block 166,determination of R_(PRTCAL)(NEW) is initiated and the value ofP_(NISTCAL) is obtained by microprocessor 22 at block 166 fromcalibrator 80 over loop 12. At block 168, microprocessor 22 measuresP_(PROBE) and R_(PRT). At block 170, microprocessor 22 calculatesR_(PRTCAL)(NEW) using Equation 7, and this value is stored in memory 30.Block 172 shows the completion of the calibration procedure and controlis returned to block 144.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, other types of references maybe used and communication of the actual values of these references maybe through any appropriate means. For example, although the calibrationprocedures described were explained with reference to two-wiretransmitters, three- and four-wire transmitters can equally well makeuse of the inventions. For example, although four-wire resistancemeasurement has been described, the concepts set forth herein areapplicable to two- or three-wire resistance measurement.

What is claimed is:
 1. A calibrator for calibrating a temperaturetransmitter in a process control loop, comprising:a NIST traceablereference; a plurality of electrical connectors adapted for electricallyconnecting to a terminal block of the transmitter; circuitry forcoupling to the process control loop and transmitting values of the NISTtraceable reference to the transmitter; and a switch connected forselectively coupling the NIST traceable reference to the plurality ofelectrical connectors.
 2. The calibrator of claim 1 wherein the NISTtraceable reference is a NIST traceable resistance.
 3. The calibrator ofclaim 1 wherein the NIST traceable reference is a NIST traceablevoltage.
 4. The calibrator of claim 1 wherein the NIST traceablereference is a temperature probe.
 5. The calibrator of claim 1 includinga NIST traceable temperature probe adapter coupling to the terminalblock and measuring a cold junction temperature.
 6. A method ofcalibrating a temperature transmitter in a process control loop,comprising:connecting an input of the transmitter to a NIST traceablereference; transmitting the value of the NIST traceable reference to thetransmitter; comparing the communicated value of the NIST traceablereference with a measured value of the NIST traceable reference; andresponsively calibrating the temperature transmitter based upon thecomparison.
 7. The method of claim 6 wherein the step of transmittingcomprises transmitting information over the process control loop to thetransmitter.
 8. The method of claim 6 including transmitting thecommunicated value of the NIST traceable reference prior to calibratingand transmitting the measured value of the NIST traceable referenceafter calibration.
 9. The method of claim 6 including:measuringtemperature of a terminal block of the transmitter; communicating themeasured temperature to the transmitter; comparing communicatedtemperature with an internally measured terminal block temperature; andcalibrating the transmitter based upon the comparison.
 10. The method ofclaim 6 wherein the NIST traceable reference is a resistance.
 11. Themethod of claim 6 wherein the NIST traceable reference is a voltage. 12.The method of claim 6 wherein the NIST traceable reference is atemperature probe.